This invention relates to electrolyte solvent and/or electrolyte additives for electrolytic cells and electrochemical devices, such as batteries, capacitors, fuel cells and displays, prepared therefrom. It further relates to novel electrolyte compositions for electrolytic cells.
Success of an alkali metal-based electrochemical devices requires the use of aprotic solvent-based electrolytes. These electrolytes must be electrochemically stable towards both cathode and anode materials. In devices such as batteries, these electrolytes must be highly conductive in order to allow useful current to be drained during use.
Current liquid aprotic electrolytes are characterized by low ionic conductivity and poor electrochemical stability. The latter leads to decomposition of the electrolyte. Decomposition products then lead to the formation of highly resistive layers at the surface of the electrodes (high interfacial impedance), which further limits the current density and useful cycle life of the device. This represents a major obstacle to the development of an alkali metal-based electrochemical device technology.
In order to overcome the difficulties inherent in liquid electrolytes, solid polymer electrolyte (SPE) materials have been developed in which ion mobility is possible through coordination of an electrolyte ion with suitable sites on the polymeric chain. Prior art polymer electrolytes have used high molecular weight polyethers (PEO) as solvents. However, PEO is crystalline at ambient temperature (20.degree. C.) with an adverse effect on the conductivity.
Gel-polymers, in which plasticizers, such as dimethoxyethane, propylene and ethylene carbonate and acetonitrile, are added to the polymer, have also been used as solvents. The lower molecular weight plasticizers are of lower viscosity and allow greater segmental motion in the polymer, thereby enhancing ion mobility in solution. Addition of plasticizer increases conductivity to a value approaching that of the plasticizer-alone conductivity and also decreases the temperature dependence of conductivity. However, most of the prior art plasticizers are unsuitable for use in alkali metal-based batteries because they have higher vapor pressures and/or are unstable with respect to alkali metals. In these respects, gel polymers are similar to aprotic liquid electrolyte systems.
Another problem which arises in current solid polymer electrolyte technology is anionic polarization in the electrolyte layer. The anions migrate during use, thereby forming a charge gradient in the electrolyte. The anions migrate only very slowly back to their original position resulting in prolonged polarization of the cell. Therefore, attempts have been made to prepare a single-ionic mobility solid polymer electrolyte in which only the cations are mobile, while the anions are fixed to the polymeric chain.
In U.S. Pat. No. 5,098,589 to Motogami et al., a solid polymer for use in an electrolyte is disclosed which is formed by cross-linking an organic compound derived from glycerol and having an average molecular weight of 1,000 to 20,000. The compound contains block co-polymer alkoxy pendant groups with substituent alkoxy groups extending therefrom. The pendant groups terminate in an active hydrogen or a polymerizable functional group. Cross-linking or polymerization of the organic compound takes place either at the polymerizable functional group or at the active hydrogen site. Conductivity at room temperature was in the range of 2.9-3.8.times.10.sup.-5 S/cm.
In U.S. Pat. No. 4,357,401 to Andre et al., a solid polymer electrolyte, a so-called "linear star polymer", based on diaminoethylene and containing block co-polymer ethylene and propylene oxide pendant groups is disclosed. Cross-linking occurs through active hydrogen sites at the pendant termini to provide a polymeric material with a conductivity in the range of 1-40.times.10.sup.-5 S/cm at 100.degree. C.
U.S. Pat. No. 5,059,443, discloses alkoxylated glucose derivatives containing fatty acid functional end groups for use as a fat substitute. The electrolytic properties of these derivatives were not investigated.
Thus far, none of the existing polymer electrolyte materials provide high ionic conductivity, high electrochemical stability and good mechanical properties at useful temperatures (preferably ambient). In addition, prior art liquid and solid polymer electrolytes do not allow for the promotion of the mobility of the ionic species of interest. This lack of control over these critical parameters greatly limits the performance, safety and cycle life of the existing alkali metal electrochemical device technologies. The present invention overcomes the above-stated limitations of the prior art.
It is the object of the present invention therefore, to provide a solid polymer electrolyte with improved conductivity and electrochemical stability and, optionally, with self-plasticizing capability.
It is a further object of the present invention to provide improved plasticizers with low vapor pressure, high conductivity and electrochemical stability and compatibility with a variety of electrolytes and cathode and anode materials.
It is yet a further object of the invention to provide an electrolytic species having high cationic mobility (high cationic transport number) and low anionic mobility.